Spent the whole weekend cleaning up my wagon model and it finally looks like a real car

That sketch thing bit me hard a couple years back. I was modeling a dirt bike frame in SolidWorks and one of my loft profiles only had half the dimensions locked in. everthing looked fine til I changed a wheelbase number upstream and the whole rear section twisted like a banana. Now I force myself to go thru the feature tree and check for the under-defined sketch flag before I close anything out. Saved me more rebuilds than I can count since. Renders are addictive once you start, fair warning. I put off learning PhotoView for ages because the shaded view felt like enough, then I tried it one weekend and suddenly I was burning hours picking between two shades of grey that look identical to everyone but me. Set a timer or you'll lose a whole evening to paint swatches. On the interior, rough blocks are honestly fine at this stage. I ran a car build for months with placeholder seats and it never bothered anyone I showed it to. Get the headlights sorted first like you said, since the front end is what the eye goes to in every render angle. Once those are real geometry you'll prolly find the seats look better through proper glass materials too, the refraction hides a lot of blockiness.

On the render thing, I acutally went the other direction with my own stuff. I render early and often now, not as a finish line treat. Back when I was modeling a kayak hull I was staring at that grey shaded view for weeks thinking the curves were clean, then threw it into a render one evening and the lighting made every ripple and crease in the surface jump out like a rash. Turns out a render with a decent studio setup is the best surface checker there is. Saves you from polishing geometry that only looks good under flat shading. I use a plain grey material for that first pass too, no paint, no gloss. Color hides sins. Once the highlights read smooth, then I let myself play with paint, and yeah I have burned a whole evening on a color picker before, it happens.

As for the lurking, two years of reading before posting is nothing. I read this place for closer to four before I put anything up, and my first post was a mess of screenshots with no context. Yours at least tells a story.

Curious how you handled the wheel move, because when I dropped the ride height on my own model the tires started clipping the arch lips at full steering lock and I had to rework the inner fender surfaces to get any sweep angle at all. Widening the offset makes it worse on the front corners too, the contact patch swings further out on turns. Also, are your body panels mirrored across the centerline or modeled as seprate bodies? I got burned doing separate lofts for each side once, the two halves were close enough that nobody noticed in the shaded view but the render lighting showed a seam right down the roofline.

I'd push back a bit on the whole 'check the under-defined flag and you're safe' thing though. A sketch can be fully defined and still be tied to the wrong edges or datums, so it drifts anyway when something upstream moves. I had a bracket on a conveyor model that was locked down tight but referenced a face that got replaced in an edit, and the rebuild threw it halfway across the part. Checking for blue lines isn't the same as checking what your references actually point at. Out of curiosity, did you fix that quarter gap by re-referencing the loft or just patching the surface over it?